The Fischer-Tropsch process (F-T synthesis) was first developed by German chemists Franz Fischer and Hans Tropsch in 1923, and this process allowed the production of liquid hydrocarbons via synthetic a gas from coal, natural gas, biomass and the like. The process of producing liquid fuels from coal is referred to as “coal-to-liquids (CTL) process”; the process of producing liquid fuels from a natural gas is referred to as “gas-to-liquids (GTL) process”; the process of producing liquid fuels from a biomass is referred to as “biomass-to-liquids (BTL) process”; and recently, the term “XTL” (“X” resource-to-liquids) is often used as a collective name for the similar processes.
These processes first convert each raw material such as coal, a natural gas, a biomass and the like into a synthetic gas through gasification, reforming and etc.; in order to produce liquid fuels, the composition of a synthetic gas suitable for a XTL process is preferably hydrogen:carbon monoxide=2:1, as shown in Reaction Formula I below, wherein CO, H2, —[CH2]—n, and H2O represent carbon monoxide, hydrogen, hydrocarbon having a chain length of n (n means the number of carbon), and water, respectively.CO+2H2+—[CH2]—n→—[CH2]—n+1+H2O  Reaction Formula I
When the ratio of hydrogen exceeds 2, this increases the methane selectivity and relatively suppresses the selectivity of C5+ (a hydrocarbon having carbon atoms of five or more), and therefore it is undesirable. Besides the linear chain hydrocarbons by Reaction Formula I, other byproducts can be produced such as olefins, oxygenates (molecules containing oxygen atom including alcohols, aldehydes, ketones, etc.) and the like.
One of the main purposes of the XTL, process is to obtain liquid fuels, and thus the current trend is to optimize catalytic reaction, ratio of synthetic gas, temperature, pressure, etc. to increase linear chain hydrocarbon selectivity, more particularly C5+ selectivity. In the catalytic reaction, a cobalt or iron based catalyst is often used, and these metal catalysts are uniformly dispersed or deposited on a support such as alumina, silica, titania and the like. Precious metals such as ruthenium, platinum, rhenium, etc. may be used as a co-catalyst to improve catalytic performance.
Meanwhile, various types of reactors can be used for the F-T synthesis, e.g., a tubular fixed bed reactor, a fluidized bed reactor, a slurry phase reactor, and a micro-channel reactor or a multi-channel reactor equipped with a heat exchanger. However, the response characteristics and the distribution of the final products may vary with the types of reactor employed, and therefore a suitable reactor should be selected depending on the final target product. A tubular fixed bed reactor, a fluidized bed reactor and a slurry phase reactor take up too much space in view of their outputs. Thus, a multi-channel reactor (covering “micro-channel reactor”) taking up a relatively small space (⅕ to ½ size of other types of reactors) for its output is recently more preferred. A multi-channel reactor is designed to maximize heat transfer efficiency so as to make it possible to run reactions at a high space velocity, and its advantages include relatively low cost of equipment and installation, convenience of easy scale-up owing to the ability to adjust systems to any desired capacity, and also mechanical loss due to friction or collision as well as loss due to changes in the reactor behavior or shaking of the catalyst, which may be caused when the equipment moves, are insignificant.
A multi-channel reactor has an alternating layered structure of catalytic beds and heat exchangers, and for the F-T synthesis, a catalyst may be loaded into the reactor by inserting the catalyst inside the reactor (i.e., a fixed-bed reactor) or attached onto the reactor by coating the catalyst on the inner wall of the reactor. In the case of coating the catalyst on the inner wall of the reactor, the loading capacity (i.e., the amount of catalyst which can be loaded in the reactor) is rather small, and thus there is a limitation on the production amount and it is very difficult or nearly impossible to replace the catalyst. Therefore, a fixed-bed reactor, which loads catalyst particles, is more preferred. In case of the fixed-bed reactor, the loading capacity of the catalyst is high and it is relatively easy to replace the catalyst. However, the heat transfer efficiency of this type of reactor is poor, and it also suffers from the formation of hot spots or run-away which makes the reaction uncontrollable.
It is very important to immediately remove the heat of reaction from the catalyst particles during the F-T synthesis because trapping of the heat of reaction may decrease the selectivity of the target hydrocarbon and causes deterioration of the catalyst. Accordingly, attempts have been made to overcome such problems by preparing a fixed-bed catalyst layer using a certain amount of an inert support (see U.S. Pat. No. 4,075,231) or mixing inert particles with the catalytic particles to form catalyst layers to properly control the reaction in conventional methods. However, in the case of using the inert support, the inert support itself is a porous material so the catalytic material soaks into the support and the catalytic reaction also takes place within the support, which makes it very difficult to control the exothermic reaction. In case of physically mixing inert particles with the catalytic particles, it is difficult to uniformly mix these particles, which causes aggregation of the catalytic material, hence preventing a uniform catalytic reaction.
Accordingly, for the synthesis extensive research has been conducted for a catalyst which has good heat transfer capability to redress such problems associated with controlling the reaction heat as well as to improve productivity. For example, U.S. Pat. Application Publication No. 2004/0251001 discloses a thin foil-type catalyst for the F-T synthesis, and KR Laid-Open Patent Publication No. 2007-0010190 discloses a catalyst having an oxidative core material, a zinc oxide shell and a catalytically active material (wherein the base material contains one or more elements selected from the group consisting of cobalt, iron, ruthenium and nickel) which is supported or coated on the shell.
Nevertheless, these conventional catalysts for the F-T synthesis failed to obtain desirable physical properties in terms of heat transfer performance.